Re: [PATCHv9 3/4] gnttab: make the grant table lock a read-write lock

[David Vrabel <david.vrabel@citrix.com>]

On 21/05/15 15:53, Jan Beulich wrote:
>>>> On 21.05.15 at 15:36, <david.vrabel@citrix.com> wrote:
>> On 21/05/15 11:32, Jan Beulich wrote:
>>>>>> On 20.05.15 at 17:54, <david.vrabel@citrix.com> wrote:
>>>> @@ -827,9 +828,11 @@ __gnttab_map_grant_ref(
>>>>              if ( (wrc + rdc) == 0 )
>>>>                  err = iommu_map_page(ld, frame, frame, IOMMUF_readable);
>>>>          }
>>>> +
>>>> +        double_gt_lock(lgt, rgt);
>>>
>>> unlock. And with this code path actually used (due to the bug it's
>>> pretty clear it didn't get exercised in your testing), how does
>>> performance look like? 
>>
>> I think it will be no worse than what it was before -- this path already
>> really sucks (mapcount() loops over 1000s of entries).  I don't care
>> about this path at all.
> 
> It's kind of strange that you don't care about this case - afaict we're
> not getting there by default solely because dom0-strict mode is not
> currently the default (albeit arguably it should be).

What's your point?  Are you going to refuse this series unless it also
optimizes dom0-strict mode?

David